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IC 9120 



Bureau of Mines Information Circular/1987 



Fuse Wire Arc Tester 



By Peter G. Kovalchik 



UNITED STATES DEPARTMENT OF THE INTERIOR 



/r ^7^&. t*» ~/i*riL 



Information Circular 9120 

M A 



Fuse Wire Arc Tester 



By Peter G. Kovalchik 




UNITED STATES DEPARTMENT OF THE INTERIOR 
Donald Paul Hodel, Secretary 

BUREAU OF MINES 
Robert C. Horton, Director 




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1 



l*> 




Library of Congress Cataloging in Publication Data: 



Kovalchik, 

Fuse wire 


Peter G. 

arc tester. 










(Information 


circular ; 9120) 










Bibliography 












Supt. of Docs 


no.: I 28.27: 9120. 










1. Fuse wire arc tester. 2. Mine explosions. 3. 
Testing. 4. Electricity in mining— Safety measures. 
(United States. Bureau of Mines) ; 9120. 


Electric 
. Title. II 


insulators and insulation- 
Series: Information circular ; 


TN295.U4 


[TN343] 


622 


s [622'.8] 


86-600152 



CONTENTS 

Pa S e 

Abstract 1 

Introduction 2 

Mechanical apparatus 3 

Electrical circuit 5 

System control 7 

Experimental method 8 

Results and conclusions 9 

ILLUSTRATIONS 

1. Comparative tracking index electrical circuit 2 

2. Comparative tracking index electrodes arrangement 2 

3. Fuse wire arc test electrodes arrangement 2 

4. Fuse wire arc test mechanical apparatus 3 

5. Drawing of mechanical apparatus 4 

6. Fuse wire arc test electrical circuit 6 

7. Mechanical drawing of specialty transformer 6 

8. Voltage setting of variable transformer for fuse wire arc tester 7 

TABLE 

1. Materials tested and test results of fuse wire arc testing program 8 





UNIT OF MEASURE 


ABBREVIATIONS 


USED IN 


THIS REPORT 


A 


ampere 




mm 


millimeter 


A-h 


ampere hour 




mV 


millivolt 


°C 


degree Celsius 




min 


minute 


ga 


gauge 




rras 


root mean square 


h 


hour 




s 


second 


in 


inch 




V 


volt 


mA 


milliampere 




W 


watt 



FUSE WIRE ARC TESTER 

By Peter G. Kovalchik 1 



ABSTRACT 

To compare the viability of the new fuse wire arc test (FWAT) as a 
substitute for the comparative tracking index (CTI) for determining sur- 
face resistance to electrical tracking, the Bureau of Mines constructed 
a fuse wire arc tester and undertook a detailed testing program for 
testing insulating materials used on explosion-proof enclosures. This 
report describes the Bureau's apparatus, the two methods (CTI and FWAT), 
and the results of the Bureau's testing, showing comparisons of the FWAT 
with the CTI. Results showed strong correlation between the two meth- 
ods, as all specimens tested that had CTI ratings of 400 V ac rras and 
above passed the 10-test sequence with the FWAT, whereas all specimens 
with lower CTI ratings failed, with the number of tests to failure cor- 
responding roughly to the descending CTI rating order. 

1 Electrical engineer, Pittsburgh Research Center, Bureau of Mines, Pittsburgh, PA. 



INTRODUCTION 



When certain insulating materials are 
decomposed by heat, highly explosive 
gases can be released. These insulating 
materials cannot be used within enclo- 
sures containing high-voltage circuits 
because they may be subjected to destruc- 
tive electrical arcing. The commonly 
used method in the United States for de- 
termining if materials are highly resist- 
ant to electrical tracking is the CTI. 
Because the CTI is a difficult test to 
perform, an alternative method, the FWAT, 



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ASTM SYMBOL KEY 

S.. Shorting switch 
J 1 Testing fixture T.. , T 2 Variable power source 

R Q Over-current relay V-. Voltmeter 

R.) Current-limiting resistor 

FIGURE 1 .—Comparative tracking index electrical circuit. 



20-mm 
minimum 



A 



60° 

1_ 



FRONT 
VIEW 




SIDE 

VIEW 



was evaluated. The CTI method subjects 
low ac voltage (up to 600 V ac rras) at 
low current to the surface of an electri- 
cal insulating material (fig. 1). This 
is accomplished by applying the voltage 
between two electrodes (fig. 2) in con- 
tact with the surface of the insulating 
material. The current results from an 
aqueous contaminant (electrolyte) , which 
is dropped between the electrodes every 
30 s. Voltage is maintained across these 
electrodes until the current flow reaches 
1 A. This value of current constitutes 
failure. Additional test specimens of 
the same material are tested at different 
voltages until failure. The results of 
these tests are graphically represented 
by plotting the number of drops required 
to cause failure versus the applied volt- 
age. From this graph, the CTI value of 
particular insulating material can be de- 
termined. The CTI is defined as the 
value of the rms voltage that will allow 
1 A to flow when the number of drops of 
contaminant required is equal to 50. 
This value provides an indication of the 
track resistance of the material. 



Electrode 



Electrode 




FIGURE 2.— Comparative tracking index electrodes arrange- 
ment. 



FIGURE 3.— Fuse wire arc test electrodes arrangement. 



The alternative FWAT method subjects 
the surface of an electrical insulating 
material to 500 V ac peak (354 V ac rras) 
and 100 A dc. This is accomplished by 
applying the alternating-current voltage 
and high direct current across two elec- 
trodes, located as shown in figure 3. 

A copper wire is sandwiched between two 
pieces of the test specimen with the ends 
of the copper wire connected to the two 
electrodes. When power is applied to the 
electrodes, the copper wire fuses from 
the high direct current, resulting in a 
carbon track. Immediately after fusing 
occurs, 500 V ac peak is maintained 
across the ends of the copper wire. This 
voltage then subjects the surface of the 
test specimen to electrical stress. If 
sufficient carbonization has taken place, 
alternating current will flow. The value 
of this alternating current is the cri- 
terion of the test. If 1 A ac or more 
flows, the insulating material fails the 
test. If less than 1 A ac flows, the 
test is repeated until either 1 A ac or 



more flows, or until 10 tests have been 
conducted. This method determines if the 
insulating material is highly resistant 
to electrical tracking by a pass or fail 
result. 

The CTI, ASTM Standard D 3638 077, has 
several disadvantages. It is completely 
impractical for the majority of users 
because the apparatus is not mobile and 
requires electrolyte solutions. The 
electrolyte, an ammonium chloride (NH4CI) 
solution, must be prepared accurately to 
ensure that conductivity of the solution 
remains constant. Also, the instrument 
should not become contaminated with the 
NH4CI solution. 

The FWAT, Electrical Research Associ- 
ation Report 5078:1964, is completely 
practical for the majority of users. 
It uses no electrolyte solution and re- 
quires only 240 V ac rras. The tests 
are short, simple, and are repeated on 
the same pair of specimens at inter- 
vals of 1 to 2 min. 



MECHANICAL APPARATUS 



A photograph of the apparatus is shown 
in figure 4, and a detailed mechanical 
drawing is shown in figure 5. The main 
design feature is the heavy terminal con- 
struction. These brass terminals provide 
an excellent electrical connection and 
minimize local heating. Also, there 
is a provision for vertical movement, 
which enables the fuse-wire connection 
to be made at the test specimen. This 
will enable tests on specimens up to 
1-1/2 in thick. 

The clamping arrangement is a simple 
design. It is only required to prevent 
movement of the specimen. The front and 
top of the clamping arrangement are 
hinged to facilitate easy access to the 
test specimen. A 20-ga spring steel wire 
is placed parallel to and 3/4 in away 
from the fuse wire. To prevent the fuse 
wire from moving when the high current is 
applied, the clamping screw applies pres- 
sure through the 1/2-in asbestos-free 
sterling board midway between the fuse 
wire and steadying wire. 



Electrode 



lectrode 




FIGURE 4.— Fuse wire arc test mechanical apparatus. 



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-Vertical hold pins 
PLAN VIEW 



O 



O 



Mounting base 
plate 




PICTORIAL 
VIEW 






Terminal post 




FRONT VIEW SIDE VIEW 

FIGURE 5.— Drawing of mechanical apparatus. 



ELECTRICAL CIRCUIT 

The electrical circuit (fig. 6) con- that is shown in figure 7. These parts 
sists of commercially available parts, consist of: 
except for a specialty transformer (T2) 

(Bl) battery: 12-V lead acid 

(BC1) battery charger: 12 V, 50 A«h 

(CR1, CR2) time-delay relays: 0-120 s, 12 V dc - coil 

(CR3) dc contactor: 220 to 250 V dc, 135 A, 2 poles 

(CR4) battery relay: 12 V dc - coil 

(Fl) 20-ga wire 

(IA1) isolation amplifier: gain = 100 

isolation = 1,500 V dc 
(LI) 220-V lamp 

(L2) 12-V lamp 

(Ml) voltmeter: 0-500 V ac rms 

(M2) battery-charger meter: 0-10 A 

(M3) ammeter: 0-200 mA ac 

(PS1, PS2) power supplies: 12 V dc, 1 A 

220 V dc 

(Rl) current-limiting resistor: 20 to 25 ohms 

3,000 to 4,000 W 
(R2) shunt: 100 A, 200 mV 

(SI, S2, S3) power switches: 240 V ac, 30 A 

(S4) micro switch: 1 pole 

(Tl) variable transformer: input 240 V ac rms 

output to 280 V ac rms 

(T2) specialty transformer 

(T3) filament transformer: 1 to 18.5 ratio 

(T4) step-down transformer: 2 to 1 ratio 



220 V ac ; 




FIGURE 6.— Fuse wire arc test electrical circuit. (See section "Electrical Circuit" for explanation of symbols.) 




V 



# 






J 



[— 5.62 ,; — ) 

8.25= — ■ 

0.56- by 0.62-in mounting slots 

FIGURE 7.— Mechanical drawing of specialty transformer (T2). 

The essential requirements of the elec- 
trical circuit and its components are 
that it should be capable of causing a 
current of approximately 100 A dc to flow 
in 20-ga copper wire and also produce a 
voltage of 500 V ac peak across the ends 



of the wire immediately after fusing 
occurs. 

A lead acid battery of 50 A* h will sup- 
ply sufficient current to fuse the wire. 
The follow-up alternating current voltage 
is obtained with a specialty transformer 
(fig. 7) designed with a 250-V ac rms 
primary and a heavy-duty secondary wind- 
ing, which provides a voltage of 500 V ac 
rms. A current limiting resistor (20 to 
25 ohms), along with the variable trans- 
former, will reduce the secondary of the 
specialty transformer to 500 V ac peak. 
It is critical for the secondary circuit 
to have a direct-current resistance less 
than 0.1 ohm to ensure the battery 
will be capable of delivering sufficient 
current. 

The remaining components of the elec- 
trical circuit are explained in detail in 
the system-control section. 



SYSTEM CONTROL 



The fuse wire arc tester must be pow- 
ered by 240 V ac rras through the standard 
240 V ac plug provided. Before energiz- 
ing the unit, make sure all switches are 
in the off or neutral position, the volt- 
age control is at zero, and the safety 
shield is down. This is to prevent any 
possible electrical shock. After com- 
pleting these steps, the control sequence 
can begin. 

Place the test specimens in the mechan- 
ical apparatus with the 20-ga copper wire 
in position. Then raise the safety 
shield up to activate micro switch S4. 
This will supply power to relay CR2 , 
which allows power to the brass terminals 
if all other switches are in the correct 
position. Switch SI is then closed and 
power is applied to all power supplies. 
Then switch S2 is closed to activate 




FIGURE 8.— Voltage setting of variable transformer (T1 ) for fuse 
wire arc tester. 



relay CR4, which then activates the bat- 
tery-charging circuit. Meter M2 will 
indicate if the battery is sufficiently 
charged. If it is not, time must be 
taken to allow sufficient charging. Once 
the battery is charged, switch S2 is 
closed to apply power to the specialty 
transformer (T2). 

The variable transformer (Tl) is ad- 
justed so the voltmeter (Ml) reads 354 
V ac rras (500 V ac peak, figure 8). When 
the specimens and fuse wire are in posi- 
tion and the correct voltage is present, 
a test can be conducted. Switch S3 is 
closed, which enables a time-delay relay 
CR1 . The time-delay relay can be set 
anywhere from to 120 s, allowing the 
operator to move a few feet away from the 
apparatus and still be able to watch the 
meters and avoid flying sparks. The con- 
tacts of relay CR1 close and activate the 
direct-current contactor (CR3). The 
direct-current contactor (CR3) closes, 
allowing 100 A dc to flow through the 
fuse wire and saturate the transformer 
(T2) core. At the same time, 240 V ac 
rms are applied at the primary of the 
specialty transformer (T2) , causing cur- 
rent to flow. This current is limited to 
10 A ac because of the 20- to 25-ohms 
current-limiting resistor (Rl). The cop- 
per wire fuses and the direct-current 
field in the transformer (T2's) core 
collapes. As a result, energy is dis- 
sipated in an arc on the surface of the 
test specimen. At the same time the pri- 
mary reactance increases, transformer 
(T2) assumes its normal operation with 
voltage across the ends of the fuse wire 
rising to 500 V ac peak. This 500 V ac 
peak stresses the surface of the speci- 
men. If sufficient carbonization has 
taken place, meter M3 will read this cur- 
rent and, if it exceeds 1 A, the specimen 
fails and the test is concluded. If the 
current is less than 1 A, another test 
must be conducted on the same specimen. 
The testing is repeated at l-l/2-min in- 
tervals until either conduction of 1 A or 
more occurs, or 10 wires have been fused. 



EXPERIMENTAL METHOD 



Objective . - To determine the resist- 
ance to arc conduction of insulating ma- 
terials. The materials tested are listed 
in table 1. 

Test Specification ; Test specimens 
consisted of two pieces, 6 by 2 in. All 
pieces were flat and free from surface 
defects. 

Conditioning: All specimens were con- 
ditioned in an environmental chamber, 
Tenny model BTH202000, as stated below: 

1. Temperature, 20° + 2° C. 

2. Humidity, 65%±5%. 

3. Time, 18 to 24 h. 

4. Time to test after removal, 3 rain. 

5. Test method for FWAT procedure: 

A. All switches in off or neu- 

tral position and safety 
shield down. 

B. Plug fuse wire arc tester 

into 240 V ac line. 

C. Place test specimen in me- 

chanical apparatus with fuse 
wire in position. 



D. 

E. 
F. 



G. 



H. 



I. 



K. 



Place safety shield up. 
Turn main-power switch on. 
Turn battery-charger switch 

on and check ammeter to see 

if battery needs charging. 
Turn same switch, but in 

opposite direction, for ac 

voltage. 
Adjust voltage control for 

354 V ac rms, as shown in 

figure 8. 
Turn test switch on and step 

back several feet. 
Read and record alternating 

current. 
If alternating current of 1 A 

or more flows, the material 

fails. 



If alternating current is less than 1 A, 
repeat the procedure on the same spec- 
imen until 1 A flows or 10 tests are 
completed. 



TABLE 1. - Materials tested and test results of fuse wire 
arc testing program 



Material 



Tradename 



Type 



Description 



CTI value, 
V 



FWAT tests 
to failure' 



RF1002. 
RF1008. 

64A 

PF1006. 
8202... 
8231.., 
R200... 
M340... 
909 



DR48. 



RTP204FR. 
420 



Nylon 6/6. 
Nylon. 
Nylon 6/6. 
Nylon 6. . . 
. . .do 



.do. 



Nylon 6/6. 

...do 

...do...., 



Polyester. 



Nylon 

Polyester. 



RTP303 Polycarbonate 



Glass flame retardant. 
NA 



NA 

Glass reinforced. 
NA 



14% glass fiber. 
NA 



Flame retardant 

Glass reinforced, 

flame retardant. 
Flame retardant 30% 

glass fiber. 
Glass reinforced. . . . 
Flame retardant 30 

glass reinforced. 
NA 



>600 
>600 
>600 
525 
475 
475 
435 
290 
255 

235 

200 
185 

150 



Passed 
Passed 
Passed 
Passed 
Passed 
Passed 
Passed 
10 
4 



NA Not available. 

'10 total tests; those that "pas 
actual number of tests to failure 
tests. 



sed" did not fail within 
was not determined for 



this testing series, but 
those that passed the 10 



RESULTS AND CONCLUSIONS 



Test results showed that the FWAT re- 
sults are comparable with those of the 
CTI. Specimens tested had CTI ratings of 
400 V ac rms and above and 290 V ac rms 
and below; those having ratings of 400 
V ac rms and above passed while those 
with 290 V ac rms and below failed. It 
appears from these results that as the 
CTI values increased from 150 to 290, the 
number of tests required to make the 
specimen break down or fail also in- 
creased. This is understandable, because 
as more FWAT tests are conducted, more 
carbonization forms on the test-specimen 
surface. When sufficient carbonization 



has taken place, more current will flow 
and eventually break down will occur. By 
examining CTI values, we know that as 
this value increases, the surface resis- 
tance to arcing increases. 

In summary, higher surface-resistance 
values need more carbonization for cur- 
rent to flow; as the CTI value Increases, 
the number of FWAT tests increase. Since 
the FWAT uses 354 V ac rms to electri- 
cally stress the surface of the specimen, 
it can be expected that CTI values 
greater than or equal to this voltage 
will pass as proven by the test results. 



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INT.-BU.0F MINES,PGH.,PA. 28394 



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Bureau of Mines— Prod, and Distr. 
Cochrans Mill Road 
P.O. Box 18070 
Pittsburgh. Pa. 15236 



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